In a papermaking process, a moist paper web, after passing through mechanical water removal stations and a series of press nips, continues to pass over a series of heated cylinders that evaporates water form the web to approximately 95% dryness. The paper web, in a typical dryer section, is unsupported as it travels in a serpentine fashion between upper and lower dryer cylinder arrangements. A top and bottom fabric loop is arranged to guide the web around the cylinders. A top fabric or felt, guides the web around the group of top cylinders, which are situated in a single row at a high elevation, with felt turning rolls located between them and a lower fabric or felt run is similarly situated at the lower level. The web travels alternately between top and bottom cylinders. The top fabric or felt run disengages at the above location from the cylinders and the paper web, continuing to a fabric or felt turning roll, passing through a series of cylinders and felt rolls until the fabric is guided above the dryer section via turning rolls of a guide and stretcher arrangement in a closed loop fashion. A similar arrangement is fashioned for the bottom dryer section, creating an opposite fabric loop system for the lower dryer group.
The stretch or distance that the moist paper web has to travel from the upper dryer group to the bottom dryer group, unsupported by either cylinder or fabric, is called the open draw.
The conventional process through the industry is to include the supply air via different pocket ventilation systems using blow boxes of many and varied systems behind the fabric or felt turning rolls (i.e. above top felt turning rolls and below bottom felt turning rolls). The air from these blow boxes has to travel through the fabric in order to infiltrate the pocket of the open draw. This requires high open permeability of the fabric and, at high machine speed, much of the air is deflected, requiring a high air volume and high horsepower to force air through the fabric. Another standard method is to introduce air via the felt turning rolls, forcing air through the fabric into the pocket.
In either of the conventional cases, the air from the supply thereof is infiltrated into the pocket through the fabric itself and the ventilating felt roll systems as described above.
It is an accepted fact that if too much air is introduced into the pocket, the web bulges causing sheet breaks, especially where the sheet has a high moisture content and is consequently very weak. In addition, edge flutter may occur, especially at high machine speeds where the edge might stretch, causing a wrinkling effect in the web. Also, introducing an air jet directly perpendicular to the unsupported paper web can blow the sheet away from the jet, causing web breakage.